1. Field of the Invention
The present invention relates to a multiprocessor system provided with shared buses for a plurality of processors, and particularly relates to a shared bus control system for effective utilization of the shared buses.
2. Description of the Related Art
Conventionally, a shared bus in a multiprocessor system has been controlled with setting a priority level for the shared buses at each of the processors. For contention of bus access between two processors, for example, the processor with the lower priority is forcibly held so that the processor with the higher priority occupies the shared buses.
Thus, conventional shared bus control for a multiprocessor system holds the processor with the lower priority and allows the one with the higher priority alone to use the shared processor upon bus contention. This means that the processor with the lower priority requires transition time to enter HOLD status and to return from HOLD status. In the case of .mu.PD70108 from NEC Corporation, for example, the HOLD ACK is returned at least two states after HOLD request sampling for setting of HOLD status. This period for transition is lost time where no operation can be made for the processor set to HOLD status.
In addition, since status transition time is variable, it is difficult to pass the bus occupation right to the processor which wants to access the memory via the shared bus always at fixed intervals.